The invention relates to a method and a device for the correction of measured value falsifications in invasive pressure measurements with a fluid-filled system, in which the measured pressure is passed via the fluid-filled system to an external pressure transducer, which converts the pressure signal into an electrical signal.
Fluid-filled systems have been used for decades in connection with invasive pressure measurement for intravenous and intraarterial pressure measurement. Such systems, also referred to as catheters, are frequently used in invasive cardiology, intensive medicine and in anesthesia, where they are used for exact pressure measurement. Use is particularly appropriate for impedance measurements on the arterial system of vessels or for derivatives of pressure with respect to time (dp/dt) for measuring the isovolumetric force of contraction or relaxation disorders of the ventricles. For this purpose, it must be possible to analyze resonances of the original pressure signals of up to approximately 30 Hz faithfully with respect to the original, that is faithfully with respect to phase and amplitude.
In invasive catheter diagnosis, the pressure measurement at a specific location in the circulation takes place via a fluid-filled system with a pressure transducer applied externally (i.e. outside the patient's body). Depending on the length, cross section, setup and elastic material properties of these systems, different resonances, attenuations and energy losses of the input pressure signal occur at the tip of the catheter.
U.S. Pat. No. 4,232,373 discloses a correction method for measurement data of a fluid-filled cardiac catheter, in which the periodically recorded signal is converted into an electrical signal, digitized and branched. Part of the signal is first passed to a correction unit and subsequently passed to a filter, while the other part is passed to the filter in an uncorrected form and with a delay. In the filter, the two parts of the signal are brought together and the corrected signal is output.
In the manuscript "Characterization of laser-induced pressure transients by means of piezoelectric PVDF-films" by S. Lohmann et al; Proc.SPIE 2624; 83-92; (1995), there is described, inter alia, the correction of laser-induced pressure waves in piezoelectric films. In this case, a description is given of the correction of a voltage signal emitted by the film by means of a Fourier transformation, in which the signal is transformed into the frequency domain and is corrected in the frequency domain by means of a correction value calculated in an algorithm. Subsequently, an inverse transformation into the time domain is carried out.
To avoid falsifications along the transmission path, the pressure transducer has been integrated into the tip of the catheter and the converted signal led out of the body via an electric line. This solution is known as a tip pressure sensor catheter. A disadvantage of this form of pressure measurement is that tip pressure sensor catheters are very expensive and have only a very restricted range of variations with respect to shape and size. Therefore, it has only been possible for this solution to be established in the scientific sector to a limited extent.
A further possible way of compensating for measured value falsifications is to consider the system as a simple forced oscillation in the physical sense and to carry out a correction of the transmission function of the system of the 2nd order after determination of the resonant frequency and the attenuation coefficient by means of an analog electric circuit or a corresponding numerical algorithm. The disadvantages of this approach are that the consideration as a system of the 2nd order is a great simplification of the actual physics of the system, in which multiple resonances can occur particularly in the case of relatively complex systems. The transmission function is, in principle, to be newly determined for each actual system, even when there are customary and frequent changes such as exchanging the catheter in the system, it being problematical to determine the transmission function by means of a flushing test or square-wave test on the patient. The transmission function is, furthermore, dependent on the elasticity of the system and this in turn is dependent on the filling pressure, the gases dissolved in the fluid and material properties of the system. Finally, these systems are very complicated to operate.
A further procedure introduced on the market is the use of systems which have been specially configured and optimized in terms of fluid mechanics by in-vitro test studies and which comprise a pressure transducer, a tube, a three-way cock, an array of cocks, a catheter and possibly an attenuator. A disadvantage of this method is that the test effort is very great and that, in invasive cardiology, an extremely wide variety of systems are used, limiting the use of this method. Furthermore, it is not possible for this attenuation to be switched off to exclude an attenuation by blood or air in the system. What the catheter personnel are accustomed to seeing makes them associate attenuation with an inadequately flushed system and they would easily misinterpret such an attenuated system.
The object of the present invention is to provide a method and a device for invasive pressure measurement with fluid-filled systems which are improved with respect to the correction of measured value falsifications, are cost-effective and versatile in their use.